1. Field of the Invention
The present invention relates to an electromagnetic disturbance power (EDP) checker, and more particularly to an EDP checker for measuring high-frequency electromagnetic disturbance generated from a small electric appliance such as an electric tool having a power cord or a main lead.
2. Description of the Prior Art
Electromagnetic disturbance waves having frequencies of 30 MHz or more are radiated from a small electric appliance. As methods of measuring the level of such radiation, there has been known an electric field strength measuring method and an absorbing clamp method. In the former method, the disturbance waves radiated from an electric appliance (hereinafter referred to as "EUT" as an abbreviation for "equipment under test") are measured directly using an antenna. In the latter method, electromagnetic disturbance waves radiated from a power cord is measured using an absorbing clamp. The absorbing clamp method is adopted to regulate the disturbance waves.
There has been known problems with directly measuring the electric field strength using the electric field strength measuring method in that the site where measurements are taken must have few external electric waves and must be an ellipsoidal area with a longest radius that is two times the measuring distance and a shortest radius that is 3.sup.1/2 times the measuring distance. Further, a reception antenna must be installed to be raised and lowered in a range from one to four meters in order to maximize the level of the received electric field strength. There has been known another problem in the electric field strength measuring method that repeatability is poor and the measured value obtained from measuring EUT varies greatly by the way the power cords are handled.
Experience has shown that almost all radiation of disturbance waves from EUT comes from the electric power cord. The absorbing clamp method is developed based on this fact and is different from the electric field strength measuring method in that the test site need be only about two meters high by seven meters wide by three meters deep. Also, repeatability of measurements in the absorbing clamp method is excellent. However, a test site with few external electric waves is necessary.
FIG. 1 shows the arrangement of equipment used in the absorbing clamp method and FIG. 2 shows the structure of an absorption clamp which is one of the essential equipment used in the clamp absorbing method. As shown in FIG. 1, measurement by way of the absorbing clamp method is performed in a radio wave shield room 20, with the above-described dimensions, for negating influence from reflection of disturbance waves radiated from EUT and influence of external disturbance waves. Typically, the EUT 5 is positioned about one meter from the metal floor and separated from an adjacent wall surface by 45 cm or more. An electric power cord 12 connected to EUT 5 is stretched tight about one meter above the floor and clamped with the absorbing clamp 19. The output from the power cord 12 is sent through a coaxial cable 13 to measuring equipment 6 which is typically a receiver. A frequency of the electromagnetic wave to be measured is set in the measuring equipment 6. The absorbing clamp 19 is moved along the power cord 12 toward and away from the EUT 5 while clamping the electric power cord 12 until a maximum level for the set frequency is indicated by the measuring equipment 6. Measurements are taken at a position where the measuring equipment 6 indicates the maximum level.
As shown in FIG. 2, the absorbing clamp 19 includes a very high frequency (VHF) current probe 2 which clamps the electric power cord 12, a row of toroidal ferrite cores 21 for use in clamping the electric power cord 12, and another row of toroidal ferrite cores 22 for use in clamping a coaxial cable 13 connected to the VHF current probe 2 for deriving the output of the VHF current probe 2. The row of ferrite cores 21 are provided for absorbing disturbance waves supplied to the power cord 12 from the EUT 5. The row of ferrite cores 22 are provided for absorbing a surface current flowing in the coaxial cable 13.
To measure the disturbance power, the absorbing clamp 19 is moved until the level indicated by the measuring equipment 6 is maximized. The position of the absorbing clamp 19 where the level of the disturbance power is at maximum corresponds to the highest level of the disturbance waves supplied from the EUT 5 to the power cord 12. The length of power cord 12 between the EUT 5 and the absorbing clamp 19 is represented by the sum of the power cord length corresponding to the electric length for one quarter the wavelength of the measuring frequency and the power cord length corresponding to the electric length for the impedance of the EUT 5.
According to the absorbing clamp method described above, the power cord 12 must have a length at least equal to the sum of one quarter the wavelength of the measuring frequency, the electric length corresponding to the impedance of the EUT 5 (i.e., one quarter the wavelength of the maximum measuring frequency), a length needed to confirm that the maximum value was exceeded, and the length of the absorption clamp 19. For example, about seven meters long power cord 12 is necessary to measure 30 MHz disturbance wave, thus a measuring site with dimensions to accommodate the seven meters long power cord 12 is necessary.
Because the length of the row of ferrite cores 21 clamping the power cord 12 is not variable and the position of the row of ferrite cores 21 is fixed relative to the position of the VHF current probe 2, the absorbing clamp 19 is moved for the purpose of maximizing the level of the disturbance power and such a long measuring site as about seven meter long in the case of measuring 30 MHz disturbance wave is necessary. Treating the power cord 12 as a distributed constant circuit, the impedance of the terminal load in the distributed constant circuit can not be changed and a current is measured at a position just before the terminal load, so the measurement is performed upon matching dependent on the length of the power cord 12 between the EUT 5 and the absorbing clamp 19.